Solar cell efficiency is a strong function of minority carrier lifetime, since photo-generated carriers that recombine before reaching the p-n junction do not contribute to photocurrent. Grain boundaries in polycrystalline silicon films provide electron traps that act as recombination centers that reduce minority carrier lifetimes. This recombination is a function of the grain boundary structure. In particular, the high dislocation density of high-angle grain boundaries result in a higher recombination rate than low angle grain boundaries. It is known that the effective carrier lifetime increases as the dislocation density decreases, and it has been shown that recombination is a strong function of grain boundary defect density.
Grain boundaries can be described as having both out-of-plane and in-plane misorientation known as tilt and twist, respectively. Both types of misorientation result in defect densities that lead to recombination. The degree of tilt and twist in a thin film grain boundary population reflects the crystallographic texture of the film. Biaxial texture, which has a preferred crystallographic direction for both out-of-plane and in-plane directions, can decrease both twist and tilt misorientation between grains. One way to develop biaxial texture is by application of an ion beam during the initial stages of nucleation of a thin film. This ion beam assisted deposition (IBAD) process uses a low energy (<1 keV), typically inert (Ar+) ion beam to develop in-plane texture in a growing thin film during concurrent physical vapor deposition of the desired source material. The ion beam is aligned along a particular crystallographic direction at an oblique angle relative to the desired out-of-plane growth direction. The ion beam sputters away unfavorably orientated crystallites and allows favorably orientated crystallites to survive and grow. If the correct channeling angle is selected then biaxial texture can develop.
The IBAD process has been used to form MgO template layers for seeding crystallographic texture in the high temperature superconductor YBa2Cy3O7, (YBCO), as its superconducting properties are dependent upon the amount of in-plane alignment. Typically, IBAD MgO can be deposited with an in-plane texture of 5-6° phi-scan FWHM and out-of-plane omega-scan FWHM of about 1°, which is very near single crystal quality.
An IBAD MgO template layer has been used, optimized for high-temperature superconductor coated conductors, as a template layer for the deposition of polycrystalline silicon. Silicon films deposited on this template layer have reduced grain boundary misorientation and increased carrier mobility. It has been shown that germanium films deposited on the MgO template layer with a CeO2 buffer layer exhibits strong biaxial texture.
Crystal silicon (c-Si) is a nearly ideal photovoltaic (PV) material: it can be highly efficient, is naturally abundant and is environmentally benign. However, silicon wafer fabrication is expensive and energy intensive, limiting potential silicon PV cost reductions. This has motivated research into ‘film crystal silicon’ PV, where c-Si is deposited directly from SiH4 onto an inexpensive substrate, thereby bypassing costly wafer fabrication steps. Such inexpensive substrates (e.g. display glass) are generally amorphous or polycrystalline and cannot sustain high temperatures for long periods; therefore, it is difficult to grow high quality c-Si on them. The key to film c-Si PV is achieving an adequate minority carrier diffusion length (LD), despite the temperature limitation. Specifically, LD must exceed three times the film thickness. Most proposed routes to film c-Si on glass result in polycrystalline films; for example, the c-Si formed by annealing amorphous silicon (a-Si) yields micron-size, randomly-oriented grains. The grain boundaries (GBs) in crystallized a-Si films have high recombination activity, reducing both LD and the solar cell open circuit voltage (VOC). Furthermore, grain boundaries parallel to the surface may impede hydrogen diffusion into the film, reducing the efficacy of post-growth hydrogenation treatments. Thus, the requirement for LD will likely require films with large columnar grains to reduce the number of GBs or well-oriented columnar grains with low-angle GBs that may be less recombination active.
To improve the crystalline order in the deposited c-Si film, researchers are investigating various ‘seed and epitaxy’ techniques. In these approaches, a seed layer with desirable grain structure is fabricated first and then the active silicon layer is grown epitaxially on that seed. For example, solar cells were demonstrated using large grained silicon seed layers formed by aluminum-induced crystallization of amorphous silicon. There has also been progress using cube-textured foil substrates as the seed, where biaxially textured Ge layers were fabricated and a proof-of-concept 1% efficient biaxially textured Si solar cell has been reported.
Although solar cells made from silicon wafers dominate the existing photovoltaic (PV) market, the wafer fabrication process is energy-intensive and expensive, comprising about half of the typical module price. Despite their high cost, silicon wafers are employed because they have excellent crystal quality and few impurities.
What is needed is a method and device with PV-quality film of crystal silicon (c-Si) on a low-cost substrate, where such “film crystal silicon” can retain the proven qualities of crystal silicon PV such as high solar conversion efficiency, safe and abundant raw materials, and high-yield manufacturing, but at a much lower cost.